Epidemiologic Concept and Models

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Epidemiologic Concept and Models

The scope of epidemiology

The definition of epidemiology given in the previous chapter, the study of the distribution and

determinants of diseases and injuries in human populations, is quite inclusive. For many years the

proviance of epidemiology was generally considered to be restricted to the infectious diseases, an

understandable focus since the major scourges of man in the past were epidemics had come under

some satisfactory control did investigation shift toward the diseases of later lifethe vascular diseases,

arthritides and malignant neoplasms.

However, even centuries ago some epidemics were traced to noninfectious causes. In the later

half of the eighteenth century colic among cider drinks in Devonshire, England,was traced to intoxication

with lead ( Beker 1967). This developed because the cider in that area was transported in leaden pipes

or processed in presses which contained lead. Due to the acidity of the cider, enough lead was leached

into the liquid to cause acute lead toxicity.

Another example of application of epidemiology to a noninfectious disease relateds to scurvy. The

occurrence of scurvy in endemic proportions on board british ships during long sea voyages was

correctly traced to nutritional deficiency (Lind 1753) which could be prevented by a ration of illness. It is

worth nothing that effective prevention did not depend on a full understanding of the dietary lack;

vitamin c was not isolated until 1928.

It is now generally accepted that epidemiologic study can appropriately be applied to all diseases,

conditions and health related events, including mental illness, suicide, drug addiction and injury.

Further, since epidemiology aims for a comprehensive view of the dynamics of disease, it is concerned

not only with epidemics but also with interepidemics periods and with sporadic and endemic occurancesof disease. Endemic occurances is defined as the habitual presence of the disease or infectious agent

within a geographic area or the usual prevalence of a given disease within such area. The term is

used in contrast with epidemics, the occurance in a community a region of a group of illness.of similar

nature, clearly in excess of normal expectancy ( Benenson, 1970).

Examination of this definition indicates thet the term epidemic itself is defined quite broadly.

1. It includes any kind of disease (or injury)2. There is no universally applicable number of cases which constitutes an epidemic. Rather, an

epidemic exists whenever the number of cases exceeds that expected or the basis of past

experience for a given population. Clearly this level of expectation varies for different diseases

and in different circumstance. In the united states in the last past 25 years one case of

smallpox would exceed expectancy, whereas the occurrence of 100 cases in a single year

might be under the expected number in Ethiopia or India

3. There is no specification of geographic extend; an epidemic may cover a few city blocks or anentire nation, or may even be worldwide in distribution, as in true in pandemics of

influenza.


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4. An epidemic may encompass any time period; it may last a few hours (chemical intoxicationor bacterial food poisioning), a few weeks (influenza or hepatitis), or several years ( drug

addiction). Many countries, including the united states, have been experiencing a lung cancer

epidemic for the past forty years.

Variations in Severity of Disease

The discussion of natural history of disease in chapter 1 outlined progression of disease

primarily in terms of chronic disease. At this point, we will shift our focus to acute short term

disease, particulary infectious disease, and consider the consequences of the fact that

infectious processes can result in variety of effects, ranging from no clinically detectable

disease to fulminating symptoms and death.

Infections differ in characteristic, or modal, severity of manifestation. Some infections ten to

be mostly inapparent; others typically


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The small fraction of infection which cause obvious disease or severe symptoms will come to

medical attention; an even smaller fraction will find their way into hospital records.

Therefore, statistics on infections with this kind of gradients are likely to be in accurate. The

number of infections diagnosed and reported will be smaller than the true number, i.e,

understated, while the severity of the disease will be over stated. Thus, where the

discrepancy between infectious and disease is large, it is particulary important to know

the criteria used for diagnosis. Many more infection will be recorded if those detected by

laboratory methods or skin tests are included than if only clinically diagnosed cases are

tabulated.

A frequently used measure of the severity of a severity of a disease is the case fatality rate,

which indicates the probability of death among diaognosed cases.

Models of Disease multiple Causation

In medicine we focus on man and the focus within him and within the environment which

influence his state of health. From this viewpoint man is the host organism; other organism

are considered only as they relate to human health. In veterinary medicine the term host

might refer to cats, dogs, or horses. However, many of the principles presented in this text

apply equally to the study of health and disease in human and animal populations.

The egocentrisms implicit in mans view of the universe has been noted in the following

poignant paragraph from zinssers


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People. Those who escaped included abortive cases, patients who died without a definitive

diagnosis and some persons who either had had the disease previously in Denmark or had

receive prophylactic gamma globulin. Only five unprotected persons in the entire population

appeared to have escaped infection entirely.

An additional set of factor, environment conditions, also determines whether effective

transmission of disease can occur in any given situation. These factors include degree of

contact, level of hygienic practices and presences of other organism.

When a factor must be present ( a sine qua non) for a disease to occur, it is called the agent of

that disease. For example, influenza virus is the agent of influenza. Many, but not all, of the

knows agents of disease are located in the biological environment. Examples of agents from

the physical environments are lead, asbestos, beryllium, carbon monoxide in the inspired air

and ionizing radiation. A possible agent in the social environments is maternal deprivation.

Numerous studies have demonstrated that the quality of parental care in the early year of life

is intimately related to normal physical, emotional, and mental development (bowlby,1952).

In keeping with the ecological view presented above, an agent is considered to be a necessary

but not sufficient cause of disease because suitable conditions of the host and environment

must also be present for disease to develop.

It is customary to divide factors affecting the development of disease into two groups , host

factors (intrinsic) and factor in the environment (extrinsic). Host factor affect susceptibilityto

disease; factors in the environments influence exposure and sometimes indirectly affect

susceptibility as well. The interactions of these two sets of factors determinate whether or not

disease develops.

Host Factors ( Intrinsic)

The state of the host at any given time is a result of the interactions of genetic endowment

with environment over the entire life span. For some conditions the relative contributions og

genetic and environmental factors are quite clear; for others it is difficult to arrive at an

assignment of weights.

An increasing number of genetically determined factors have been identified as related to

either an increased susceptibility to certain diseases. ABO blood type is associated with

several diseases. Persons with A blood have an increased risk of gastric cancer, while those

with type O are more likelyto develop duodenal ulcer. Defi


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Death. Another infection less uniformly fatal but still very severe is produced by salmonella

choleraesuis.

The analysis of severity of infections just presented is an over simplification since it implies,

incorrectly, that variation depends on entirely of the infecting organism. In actuality, severity of

manifestation also depends on the nature of response to the organism. The importance of host factors

will be discussed later in this chapter.

Even within one genus of bacteria there can be great variability in the severity of disease

produced by different types or groups of the organism. It is well known, for example, that one specific

group of hemolytics streptococci, group A organisms, are responsible for most the streptococcal disease

of the man. Similarly,the various serotypes of salmonella differ in their tendency to produce disease.

Figure 2-2 presents data from a five year period in Poland which show that the proportion of

symptomatic infections varied widely for different serotypes, from 86 per cent for S.choleraesuis to 7

per cent for S. give.

Of course, as mentioned above, for any given serotypes the extent of reaction will also be

influenced by characteristics of the host. The same serotypes would produce symptoms in a higher


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Environmental Factors ( Extrinsic )

Extrinsic or environmental factors can be classified as biological, social and physical.

Biological environment. This sector of the environment includes (1) infectious agents of

disease, (2) reservoirs of infection ( other human beings, animals and soil) (3) vectors which transmit

disease (e.g., flies and mosquitos) and (4) plants and animals (as sources of food, antibiotics and other

drug principles or as antigents). Most of these topics will be elaborated further in chapter 12.

Social environment. The social environment may be defined in terms of the overall economics

and political organization of a society and of the institutions by which individuals are integrated into the

society at various stages in their lives. All of these factors are relevant to health. Broadly speaking,

overall socioeconomics and political organization affect the technical level of medical care, the systems

by which that care is delivered, the extend of support for medical care and biomedical research, and the

adequacy and level of enforcement of codes and laws controlling health related environmental hazards (

pollution, housing, occupational safety and so on).

Particular social customs may affect health. The types of foods eaten and the thorcughness of

cooking determine whether there will be exposure to parasites, such as fish tapeworms and trichinae.

The practice of wearing shoes can prevent acquisition of hookworms in rural areas where these

parasites are prevalent.

Another important aspect of social environment is the general level of receptivity to new

ideas. When physian and other health personal try to encourage healthful practices, resistance may

develop, at least in part, because these practices run counter to deeply held beliefs and values. This can

apply equally to problem encountered in insuring an adequate intake of milk in the diet of pregnant

Zulu women and those found in persuading Americans to use seat belts. A number of other illustration

of similar phenomena are recorded and discussed in health, culture and community.

The extend to which individuals are integrated into a society is vitally significant to health. In

general, a high degree of integration is protective; social isolation and alienation are productive of

disease. The pioneer work in this area was done by Durkheim ( 1897) in his studies of suicide. More

recently, schizophrenia (Faris and Dunham, 1939; Hare, 1965) and depressed mental develop

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A source of stressor, its positive as well as its negative aspect hear consideration. The range of

environmental conditions (e.g atmospheric pressure , oxygen supply, living space) compatible with

human existence on earth is relatively

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Epidemiologic Concept and Models